Non-woven fabric base material for lithium ion secondary battery separator and lithium ion secondary battery separator

ABSTRACT

A non-woven fabric base material for a lithium ion secondary battery separator composed mainly of a polyethylene terephthalate fiber, characterized in that the non-woven fabric base material comprises a polyethylene terephthalate binder fiber and a crystallized polyethylene terephthalate fiber, and the content of a polyethylene terephthalate binder fiber having a fiber length of 2.5 mm or less is 10 to 60 mass %.

TECHNICAL FIELD

This invention relates to a non-woven fabric base material for a lithiumion secondary battery separator, and a lithium ion secondary batteryseparator.

BACKGROUND ART

As a lithium ion secondary battery separator (sometimes abbreviated as“separator” hereafter) for a lithium ion secondary battery (sometimesabbreviated as “battery” hereafter), a porous film composed of apolyolefin resin such as polyethylene and polypropylene has beenconventionally used. However, the resin-based porous film has had theproblem that when a battery generates heat abnormally, the film meltsand shrinks, and loses a function of separating positive and negativeelectrodes, which leads to a problem of causing a serious short circuit.

As a separator that is hard to melt and shrink even when a batterygenerates heat abnormally, there has been proposed a separator producedby coating various inorganic pigments onto a polyethylene terephthalate(PET) fiber-containing non-woven fabric base material for a lithium ionsecondary battery separator (sometimes abbreviated as “non-woven fabricbase material” hereafter) (for example, see Patent Documents 1-3).

Patent Document 1 describes a non-woven fabric base material comprisinga fiber having a fiber diameter of 0.1 to 10 μm. Patent Document 2describes a non-woven fabric base material comprising a crystallized PETfiber and a PET binder fiber, in which a short fiber having an averagefiber diameter of 3 μm or less is contained as an essential component.Patent Document 3 describes a non-woven fabric base material comprisinga crystallized PET fiber and a PET binder fiber, in which a fiber havinga fiber length of 2 mm or less is contained as the crystallized PETfiber. However, each of non-woven fabric base materials disclosed inPatent Documents 1-3 has such problems that when it is intended toproduce a separator whose thickness is small, the non-woven fabric basematerial is easy to be wrinkled during coating a coating liquidcontaining an inorganic pigment, which results in a decline in theproductivity of the separator. For example, in Examples 2a to 2u inPatent Document 1, in order to produce a separator from a non-wovenfabric base material having a thickness of 13 μm, there is used acomplicatedly structured apparatus in which a non-woven fabric basematerial is conveyed while supporting said base material by a belt.Further, a coating liquid containing an inorganic pigment is coated onthe base material at an extremely low line speed of 8 m/hr. In addition,there are cases where the coating liquid may penetrate though the basematerial, or there are cases where it may be difficult to achieve bothlow internal resistance and high tensile strength.

In addition, in each of the separators disclosed in Patent Documents1-3, when the melting of a PET binder fiber is advanced to enhance thestrength of the non-woven fabric base material, the PET binder fiberfills up pores inside the non-woven fabric base material, which lead toa problem that electrolyte retention worsens and a problem that theresistance of the separator becomes high.

PRIOR ART DOCUMENTS Patent Documents

[Patent Document 1] JP 2005-536857 A

[Patent Document 2] JP 2009-230975 A

[Patent Document 3] JP 2011-82148 A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention is to solve the above-mentioned problems. That is,the present invention aims at providing a non-woven fabric base materialfor a lithium ion secondary battery separator, which is hard to bewrinkled during coating a coating liquid containing an inorganicpigment, and can provide high productivity of the separator. Further,the present invention aims at providing a non-woven fabric base materialfor a lithium ion secondary battery separator, which is hard to causethe penetration-through of a coating liquid containing an inorganicpigment during coating the coating liquid, and can achieve both lowinternal resistance and high tensile strength. Furthermore, the presentinvention aims at providing a non-woven fabric base material for alithium ion secondary battery separator, which has high strength andgood electrolyte retention, and can lower the resistance of a separator.

Means to Solve the Problems

The above problems have been solved by the following invention.

(1) A non-woven fabric base material for a lithium ion secondary batteryseparator composed mainly of a polyethylene terephthalate fiber,characterized in that:

the non-woven fabric base material comprises a polyethyleneterephthalate binder fiber and a crystallized polyethylene terephthalatefiber, and the content of a polyethylene terephthalate binder fiberhaving a fiber length of 2.5 mm or less is 10 to 60 mass %,

(2) The non-woven fabric base material for a lithium ion secondarybattery separator recited in claim 1, wherein the content of apolyethylene terephthalate binder fiber having an average fiber diameterof 14.0 μm or less and a fiber length of 0.5 to 2.5 mm is 21 to 60 mass%,

(3) The non-woven fabric base material for a lithium ion secondarybattery separator recited in claim 1,

wherein the content of a polyethylene terephthalate binder fiber havingan average fiber diameter of 1.5 to 2.8 μm and a fiber length of 1.0 to2.5 mm is 10 to 30 mass %;

the total content of the polyethylene terephthalate binder fiber and thecrystallized polyethylene terephthalate fiber is 80 to 100 massa; and

the average fiber diameter of the crystallized polyethyleneterephthalate fiber is 2.0 to 4.0 μm,

(4) A non-woven fabric base material for a lithium ion secondary batteryseparator composed mainly of a polyethylene terephthalate fiber,characterized in that:

the non-woven fabric base material comprises a polyethyleneterephthalate binder fiber containing 3,5-dicarbomethoxy benzenesulfonic acid as a copolymer component,

(5) A lithium ion secondary battery separator, produced by subjectingthe non-woven fabric base material for a lithium ion secondary batteryseparator recited in any of claims 1 to 4 to at least one treatmentselected from:

a treatment in which a coating liquid containing an inorganic pigment iscoated;

a treatment in which a coating liquid containing an organic particle iscoated;

a treatment in which a microporous resin film is laminated;

a treatment in which a fine fiber layer is formed by an electrospinningmethod; and

a treatment in which a solid electrolyte or a gel-like electrolyte iscoated.

Effect of the Invention

Since the non-woven fabric base material of the present invention ishard to be wrinkled during coating a coating liquid containing aninorganic pigment, there can be produced with high productivity alithium ion secondary battery separator in which the inorganic pigmentis coated on the non-woven fabric base material. Further since thenon-woven fabric base material of the present invention is hard to causethe penetration-through of a coating liquid containing an inorganicpigment during coating the coating liquid, there can be produced alithium ion secondary battery separator achieving both low internalresistance and high tensile strength. Furthermore, there can be produceda non-woven fabric base material for a lithium ion secondary batteryseparator having high strength and good electrolyte retention.

That is, the non-woven fabric base material for a lithium ion secondarybattery separator composed mainly of a polyethylene terephthalate (PET)fiber comprises a PET binder fiber and a crystallized PET fiber, and thecontent of a PET binder fiber having a fiber length of 2.5 mm or less is10 to 60 mass %, whereby the number of the PET binder fiber in thenon-woven fabric base material increases, and the PET binder fiber canbe distributed uniformly inside the non-woven fabric base material,which makes the base material less susceptible to be wrinkled duringcoating, and can improve the productivity of the separator.

Especially, when the non-woven fabric base material contains 21 to 60mass % of a PET binder fiber having an average fiber diameter of 14.0 μmor less and a fiber length of 0.5 to 2.5 mm, the number of the PETbinder fiber in the non-woven fabric base material further increases,and the PET binder fiber can be distributed more uniformly inside thenon-woven fabric base material, which makes the base material lesssusceptible to be wrinkled during coating, and the effect of enhancingthe productivity of the separator can be achieved.

Further, when the non-woven fabric base material contains a PET binderfiber having an average fiber diameter of 1.5 to 2.8 μm and a fiberlength of 1.0 to 2.5 mm, both of the number of the PET binder fiber andthe binding force per single fiber increase. For this reason, even at asmall content of the PET binder fiber such as 10 to 30 mass %,sufficiently high strength can be realized, and as a result, lowinternal resistance can be attained. Further, when the average fiberdiameter of a crystallized PET fiber is 2.0 to 4.0 μm, there can beobtained a suitably dense non-woven fabric base material, which canreduce the penetration-through of a coating liquid and can achieve lowinternal resistance.

In addition, when the non-woven fabric base material for a lithium ionsecondary battery separator composed mainly of a PET fiber comprises aPET binder fiber containing 3,5-dicarbomethoxy benzene sulfonic acid asa copolymer component, the fibers can bond together without excessivelyfilling up pores inside the non-woven fabric base material, and the highstrength of the non-woven fabric base material for a lithium ionsecondary battery separator can be obtained without worseningelectrolyte retention.

DESCRIPTION OF EMBODIMENTS Non-Woven Fabric Base Material (1) for aLithium Ion Secondary Battery Separator

A non-woven fabric base material (1) is composed mainly of a PET fiber,and has a characterizing feature that it comprises a PET binder fiberand a crystallized PET fiber, and the content of a PET binder fiberhaving a fiber length of 2.5 mm or less is 10 to 60 mass %.

The non-woven fabric base material (1) contains a PET binder fiber. Whenthe fiber length of the PET binder fiber is more than 2.5 mm, thenon-woven fabric base material becomes easy to elongate, or the PETbinder fiber becomes easy to tangle with each other, and thedistribution thereof inside the non-woven fabric base material becomesun-uniform. For these reasons, such cases sometimes occur that thenon-woven fabric base material becomes easy to be wrinkled duringcoating and the strength thereof becomes low. The length of the PETbinder fiber is preferably 0.5 to 2.5 mm, more preferably 0.7 to 2.3 mm,and most preferably 1.0 to 2.0 mm.

The content of a PET binder fiber having a fiber length of 2.5 mm orless is 10 to 60 mass %. When the content thereof is less than 10 mass%, the strength of the non-woven fabric base material becomes low or thenon-woven fabric base material becomes easy to be wrinkled. When thecontent thereof is more than 60 mass %, melted components fill up finepores inside the non-woven fabric base material and the electrolyteretention of the non-woven fabric base material worsens. Further, theinternal resistance of the non-woven fabric base material becomes high.The content of the PET binder fiber having a fiber length of 2.5 mm orless is preferably 15 to 50 mass %, more preferably 20 to 40 mass %, andmost preferably 25 to 35 mass %.

The average fiber diameter of the PET binder fiber is preferably 0.1 to14.0 μm. When the average fiber diameter thereof is less than 14.0 μm,the number of the fiber in the thickness direction increases, and thestrength of the non-woven fabric base material becomes high. When thePET binder fiber is too thin or fine, it sometimes falls out of thenon-woven fabric base material. Therefore, the average fiber diameterthereof is preferably 0.1 μm or more. In addition, the average fiberdiameter of the PET binder fiber is more preferably 1.0 to 13.0 μm,further more preferably 1.5 to 10.0 μm, and most preferably 2.0 to 10.0μm.

The term “average fiber diameter” mentioned here in the specificationmeans an average value obtained by measuring equivalent circulardiameter values of 20 fibers which form the non-woven fabric basematerial, from scanning electron microscopic images of the cross-sectionof the non-woven fabric base material, and thereafter averagingequivalent circular diameter values of smaller 10 fibers. The reason whyonly 10 smaller measured values are used is to exclude the measuredvalues of the fibers which are cut far from a perpendicular line to thefiber longitudinal direction.

The PET binder fiber includes a composite fiber such as core-sheathtype, eccentric core type, side-by-side type, sea-island type, orangetype and multiple bimetal type, and a single component type fiber. Inview of achieving uniformity, a single-component type heat-fusible fiberis especially preferable.

The non-woven fabric base material (1) contains a crystallized PETfiber. The content of the crystallized PET fiber is preferably 40 to 90massa, more preferably 50 to 85 mass %, further more preferably 60 to 80mass %, and most preferably 65 to 75 mass %. When the content of thecrystallized PET fiber is either less than 40 mass % or more than 90massa, the strength of the non-woven fabric base material sometimesbecomes low.

The average fiber diameter of the crystallized PET fiber is preferably0.1 to 10.0 μm, more preferably 0.5 to 9.0 μm, and further morepreferably 1.0 to 8.0 μm. When the average fiber diameter thereof isless than 0.1 μm, the fiber is so thin or fine that it sometimes fallsout of the non-woven fabric base material, and when the average fiberdiameter thereof is more than 10.0 μm, it sometimes becomes difficult tomake the separator thinner.

The fiber length of the crystallized PET fiber is preferably 1 to 10 mm,more preferably 2 to 7 mm, and further more preferably 3 to 5 mm. Whenthe fiber length thereof is less than 1 mm, the strength of thenon-woven fabric base material sometimes weakens, and when it is morethan 10 mm, the fibers entangle and form lumps, and the thickness of thebase material sometimes becomes un-uniform.

The non-woven fabric base material (1) is composed mainly of a PETfiber. In the non-woven fabric base material (1), the term “composedmainly of a PET fiber” means that the content of the PET fiber is 70mass % or more. Further, other fibers than the PET fiber can becontained. For example, short fiber or fibrillated material ofsolvent-spun cellulose and regenerated cellulose; natural cellulosefiber; pulped material or fibrillated material of natural cellulosefiber; single fiber or composite fiber of polyolefin, acrylic resin,wholly aromatic polyester, wholly aromatic polyesteramide, polyamide,semi-aromatic polyamide, wholly aromatic polyamide, wholly aromaticpolyether, wholly aromatic polycarbonate, wholly aromaticpolyazomethine, polyimides, polyamideimide (PAI), polyetheretherketone(PEEK), polyphenylene sulfide (PPS), poly-p-phenylenebenzo-bisoxazole(PBO), polybenzimidazole (PBI), polytetrafluoroethylene (PIKE),ethylene-vinyl alcohol copolymer; and fibrillated material or dividedmaterial of single fiber or composite fiber consisting of these resins.One kind or two or more kinds of these fibers can be contained in thebase material (1). The term “semi-aromatic” means that a chain such asfatty chain is partly contained in a main chain. Wholly aromaticpolyamide can be either para-type or meta-type.

The basis weight of the non-woven fabric base material (1) is preferably6.0 to 20.0 g/m², more preferably 8.0 to 18.0 g/m², and further morepreferably 10.0 to 16.0 g/m². When it is more than 20.0 g/m², itsometimes becomes difficult to make the separator thinner, and when itis less than 6.0 g/m², it sometimes becomes difficult to obtainsufficient strength. The basis weight is measured based on the methoddefined in JIS P 8124 (Paper and board—Determination of basis weight).

The thickness of the non-woven fabric base material (1) is preferably 10to 30 μm, more preferably 13 to 27 μm, and further more preferably 15 to25 μm. When it is less than 10 μm, sufficient strength of the non-wovenfabric base material cannot be obtained, and when it is more than 30 μm,it is difficult to make the separator thinner. The thickness is measuredby an outside micrometer whose resolution is 0.001 mm, as defined in JISB7502-1994.

Non-Woven Fabric Base Material (2) for a Lithium Ion Secondary BatterySeparator

A non-woven fabric base material (2) has a characterizing feature that,in the non-woven fabric base material (1), the content of a PET binderfiber (sometimes abbreviated as “PET binder fiber (I)” hereafter) havingan average fiber diameter of 14.0 μm or less and a fiber length of 0.5to 2.5 mm is 21 to 60 mass %.

The term “average fiber diameter” mentioned here in the specificationmeans an average value obtained by measuring equivalent circulardiameter values of 20 fibers which form the non-woven fabric basematerial, from scanning electron microscopic images of the cross-sectionof the non-woven fabric base material, and thereafter averagingequivalent circular diameter values of smaller 10 fibers. The reason whyonly 10 smaller measured values are used is to exclude the measuredvalues of the fibers which are cut far from a perpendicular line to thefiber longitudinal direction.

When the average fiber diameter of the PET binder fiber (I) is 14.0 μmor less, the number of the fiber in the thickness direction increases,and the strength of the non-woven fabric base material becomes high.When the PET binder fiber (I) is too thin or fine, it sometimes fallsout of the non-woven fabric base material. Therefore, the average fiberdiameter of the PET binder fiber (I) is preferably 0.1 μm or more. Inaddition, the average fiber diameter of the PET binder fiber (I) is morepreferably 1.0 to 13.0 μm, and further more preferably 2.0 to 10.0 μm.

When the fiber length of the PET binder fiber (I) is less than 0.5 mm,the fiber sometimes falls out of the non-woven fabric base material.When the fiber length thereof is more than 2.5 mm, the non-woven fabricbase material becomes easy to elongate, or the PET binder fiber (I)becomes easy to tangle with each other, and the distribution thereofinside the non-woven fabric base material becomes un-uniform. For thesereasons, the non-woven fabric base material becomes easy to be wrinkledduring coating. The length of the PET binder fiber (I) is preferably 0.7to 2.3 mm, and more preferably 1.0 to 2.0 mm.

When the content of the PET binder fiber (I) is less than 21 mass %, thestrength of the non-woven fabric base material becomes low or thenon-woven fabric base material becomes easy to be wrinkled. When thecontent thereof is more than 60 massa, melted components fill up finepores inside the non-woven fabric base material and the electrolyteretention of the non-woven fabric base material worsens. The content ofthe PET binder fiber (I) is preferably 25 to 50 mass %, more preferablymore than 30 mass % to 50 mass %, and most preferably 35 to 45 mass %.

The non-woven fabric base material (2) contains a crystallized PETfiber. The content of the crystallized PET fiber is preferably 40 to 79mass %, more preferably 50 to 75 mass %, further more preferably 50 mass% to less than 70 mass %, and most preferably 55 to 65 massa. When thecontent of the crystallized PET fiber is either less than 40 massa, ormore than 79 massa, the strength of the non-woven fabric base materialsometimes weakens.

The average fiber diameter of the crystallized PET fiber is preferably0.1 to 10.0 μm, more preferably 0.5 to 9.0 μm, and further morepreferably 1.0 to 8.0 μm. When the average fiber diameter thereof isless than 0.1 μm, the fiber is so thin or fine that it sometimes fallsout of the non-woven fabric base material, and when it is more than 10.0μm, it sometimes becomes difficult to make the separator thinner.

The fiber length of the crystallized PET fiber is preferably 1 to 10 mm,more preferably 2 to 7 mm, and further more preferably 3 to 5 mm. Whenthe fiber length thereof is smaller than 1 mm, the strength of thenon-woven fabric base material sometimes becomes low, and when it ismore than 10 mm, the fibers tangle together and form lumps, and thethickness of the base material sometimes becomes un-uniform.

The non-woven fiber base material (2) can contain a PET binder fiberother than the PET binder fiber (I), but the content thereof ispreferably 20 mass % or less. When the content thereof is more than 20massa, melted components of the PET binder fiber fill up fine poresinside the non-woven fabric base material and the resistance of aseparator sometimes becomes high.

The PET binder fiber includes a composite fiber such as core-sheathtype, eccentric core type, side-by-side type, sea-island type, orangetype and multiple bimetal type, and a single component type fiber. Inthe view of achieving uniformity, a single-component type heat-fusiblefiber is especially preferable.

The basis weight of the non-woven fabric base material (2) is preferably6.0 to 20.0 g/m², more preferably 8.0 to 18.0 g/m², and further morepreferably 10.0 to 16.0 g/m². When it is more than 20.0 g/m², itsometimes becomes difficult to make the separator thinner, and when itis less than 6.0 g/m², it sometimes becomes difficult to obtainsufficient strength. The basis weight is measured based on the methoddefined in JIS P 8124 (Paper and board—Determination of basis weight).

The thickness of the non-woven fabric base material (2) is preferably 10to 30 μm, more preferably 13 to 27 μm, and further more preferably 15 to25 μm. When it is less than 10 μm, sufficient strength of the non-wovenfabric base material sometimes cannot be obtained, and when it is morethan 30 μm, it is difficult to make the separator thinner. The thicknessis measured by an outside micrometer whose resolution is 0.001 mm, asdefined in JIS B7502-1994.

Non-Woven Fabric Base Material (3) for a Lithium Ion Secondary BatterySeparator

A non-woven fabric base material (3) is characterized in that, in thenon-woven fabric base material (1), the content of a PET binder fiber(sometimes abbreviated as “PET binder fiber (II)” hereafter) having anaverage fiber diameter of 1.5 to 2.8 μm and a fiber length of 1.0 to 2.5mm is 10 to 30 mass %; the total content of the PET binder fiber and acrystallized PET fiber is 80 to 100 mass %; and the average fiberdiameter of the crystallized PET fiber is 2.0 to 4.0 μm.

The term “average fiber diameter” mentioned here in the specificationmeans an average value obtained by measuring equivalent circulardiameter values of 20 fibers which form the non-woven fabric basematerial, from scanning electron microscopic images of the cross-sectionof the non-woven fabric base material, and thereafter averagingequivalent circular diameter values of smaller 10 fibers. The reason whyonly 10 smaller measured values are used is to exclude the measuredvalues of the fibers which are cut far from a perpendicular line to thefiber longitudinal direction.

By using the PET binder fiber (II) with the average fiber diameter of2.8 μm or less, the number of the PET binder fiber increases, thespecific surface area of the PET binder fiber becomes large, and thebinding force thereof is enhanced, which makes it possible to obtainsufficient strength even in such a small content of 30 mass % or less.However, in the non-woven fabric base material (3), even when a smallamount of the PET binder fiber with a fiber diameter of more than 2.8 μmis contained, the above effects achieved by the non-woven fabric basematerial (3) are not greatly affected. Herein, the term “small amount”means that the content thereof is 15 mass % or less to the non-wovenfabric base material.

Further, when a PET binder fiber with a small average fiber diametersuch as the PET binder fiber (II) has a fiber length of more than 2.5mm, the PET binder fibers become easy to tangle together, and thedistribution thereof inside the non-woven fabric base material becomesun-uniform, which reduces the binding force thereof. As a result, it isnot possible to obtain sufficient strength in such a small content of 30mass % or less. However, in the non-woven fabric base material (3), evenwhen a small amount of the PET binder fiber with a fiber length of morethan 2.5 mm is contained, the above effects achieved by the non-wovenfabric base material (3) are not greatly affected. Herein, the term“small amount” means that the content thereof is 10 mass % or less tothe non-woven fabric base material.

By using the PET binder fiber (II) with the average fiber diameter of1.5 μm or more, the PET binder fibers can be prevented from tanglingtogether, and the distribution thereof inside the non-woven fabric basematerial can be uniform, which enhances the binding force thereof. As aresult, it becomes possible to obtain sufficient strength in such asmall content of 30 mass % or less. However, in the non-woven fabricbase material (3), even when a small amount of the PET binder fiber witha fiber diameter of less than 1.5 μm is contained, the above effectsachieved by the non-woven fabric base material (3) are not greatlyaffected. Herein, the term “small amount” means that the content ofthereof is 5 mass % or less to the non-woven fabric base material.

By using the PET binder fiber (II) with the fiber length of 1.0 mm ormore, the PET binder fiber can be prevented from falling out of thenon-woven fabric base material, and as a result, it also becomespossible to obtain sufficient strength in such a small content of 30mass % or less. In this view, the fiber length of the PET binder fiber(II) is more preferably 1.5 mm or more. However, in the non-woven fabricbase material (3), even when a small amount of the PET binder fiber witha fiber length of less than 1.0 mm is contained, the effects achieved bythe non-woven fabric base material (3) are not greatly affected. Herein,the term “small amount” means that the content of thereof is 5 mass % orless to the non-woven fabric base material.

When the content of the PET binder fiber (II) is less than 10 mass %,the strength of the non-woven fabric base material becomes low. In thisview, the content of the PET binder fiber (II) is more preferably 15mass % or more. When the content of the PET binder fiber (II) with asmall average fiber diameter is more than 30 mass %, melted componentsfill up fine pores inside the non-woven fabric base material and theinternal resistance sometimes becomes high. In this view, the content ofthe PET binder fiber (II) is preferably 30 mass % or less, and morepreferably 25 mass % or less.

As the PET binder fiber (II), there can be used a single component typePET binder fiber staple, which is produced by firstly conductingmelt-spinning to produce a sea-island type fiber filament which consistsof a PET resin as an island component and a suitable solvent-solubleresin such as an aqueous alkaline solution-soluble polyester resin as asea component; then eluting the sea component therefrom to produce afiber with an average fiber diameter of 1.5 to 2.8 μm; and finallycutting the fiber obtained above with a suitable cutting apparatus sothat the fiber length thereof becomes 1.0 to 2.5 mm. It is also possiblethat the sea-island type fiber filament is firstly cut so that the fiberlength thereof becomes 1.0 to 2.5 mm, and then the sea component iseluted therefrom. As 1.5 long as the average fiber diameter is 1.5 to2.8 μm and the fiber length is 1.0 to 2.5 mm, there can be used acomposite PET binder fiber such as core-sheath type, eccentric coretype, side-by-side type, sea-island type, orange type and multiplebimetal type, and a single-component type PET binder fiber, all of whichare produced by other methods than the above-mentioned methods.

The non-woven fabric base material (3) contains the PET binder fiber(II) and a crystallized PET fiber in a total content of 80 to 100 mass%. In other words, other fibers than the PET binder fiber (II) and thecrystallized PET fiber may be contained, but the content thereof islimited to 20 mass % or less. When the content of other fibers than thePET binder fiber (II) and the crystallized PET fiber is more than 20mass %, the binding force between the PET binder fiber (II) and otherfibers becomes lower, which makes it impossible to obtain a non-wovenfabric base material with high strength. In this view, it is morepreferable that the non-woven fabric base material (3) contains the PETbinder fiber and a crystallized PET fiber in total content of 90 mass %or more.

In the non-woven fabric base material (3), a fiber with an average fiberdiameter of 2.0 to 4.0 μm is used as the crystallized PET fiber. Whenthe average fiber diameter thereof is less than 2.0 μm, the non-wovenfabric base material is so dense that the internal resistance thereofbecomes high. However, even when a small amount of the crystallized PETfiber with an average fiber diameter of less than 2.0 μm is contained,the above effects achieved by the non-woven fabric base material (3) arenot greatly affected. Herein, the term “small amount” means that thecontent of thereof is 15 mass % or less to the non-woven fabric basematerial.

When the average fiber diameter of the crystallized PET fiber is morethan 4.0 μm, the denseness of the non-woven fabric base material isinsufficient, and the penetration-through of a coating liquid becomeseasy to occur. In this view, the average fiber diameter of thecrystallized PET fiber is preferably 3.5 μm or less. However, in thenon-woven fabric base material (3), even when a small amount of thecrystallized PET fiber with an average fiber diameter of more than 4.0μm is contained, the above effects achieved by the non-woven fabric basematerial (3) are not greatly affected. Herein, the term “small amount”means that the content of thereof is 15 mass % or less to the non-wovenfabric base material.

In the non-woven fabric base material (3), the fiber length of thecrystallized PET fiber is preferably 2.5 to 6.0 mm. When the fiberlength of the crystallized PET fiber is less than 2.5 mm, it sometimesbecomes impossible to obtain suitable tensile strength. When it is morethan 6.0 mm, the texture worsens due to tangling of the fibers, whichmay cause a high thickness defect that has an undesirable influence onits use as a separator.

The basis weight of the non-woven fabric base material (3) is preferably6.0 to 12.0 g/m². When it is less than 6.0 g/m², it is sometimesdifficult to obtain sufficient strength. When it is more than 12.0 g/m²,it is sometimes difficult to make the separator thinner. The basisweight is measured based on the method defined in JIS P 8124 (Paper andboard—Determination of basis weight).

The thickness of the non-woven fabric base material (3) is preferably 8to 18 μm. When it is less than 8 μm, the penetration-through of acoating liquid becomes easy to occur, even in the non-woven fabric basematerial (3). When it is more than 18 μm, the internal resistancesometimes becomes high. In the present invention, the thickness of thenon-woven fabric base material is measured by an outside micrometerwhose resolution is 0.001 mm, as defined in JIS B7502-1994.

Non-Woven Fabric Base Material (4) for a Lithium Ion Secondary BatterySeparator

A non-woven fabric base material (4) is composed mainly of a PET binderfiber, and comprises a PET binder fiber (sometimes abbreviated as “PETbinder fiber (III)” hereafter) containing 3,5-dicarbomethoxy benzenesulfonic acid as a copolymer component.

The average fiber diameter of the PET binder fiber (III) is preferably0.5 to 14.0 μm, more preferably 1.0 to 13.0 μm, and further morepreferably 2.0 to 10.0 μm. When it is less than 0.5 μm, the fibersometimes falls out of the non-woven fabric base material. When it ismore than 14.0 μm, the number of the fiber in the thickness directiondecreases, the strength of the non-woven fabric base material sometimesbecomes low.

The term “average fiber diameter” mentioned here in the specificationmeans an average value obtained by measuring equivalent circulardiameter values of 20 fibers which form the non-woven fabric basematerial, from scanning electron microscopic images of the cross-sectionof the non-woven fabric base material, and thereafter averagingequivalent circular diameter values of smaller 10 fibers. The reason whyonly 10 smaller measured values are used is to exclude the measuredvalues of the fibers which are cut far from a perpendicular line to thefiber longitudinal direction.

The fiber length of the PET binder fiber (III) is preferably 0.5 to 5.0mm, more preferably 0.7 to 4.0 mm, and further more preferably 1.0 to3.0 mm. When the length thereof is less than 0.5 mm, the fiber sometimesfalls out of the non-woven fabric base material. When it is more than5.0 mm, the fibers sometimes tangle together and form lumps, and thethickness sometimes becomes un-uniform.

The content of the PET binder fiber (III) is preferably 5 to 60 mass %,more preferably 10 to 55 mass %, and further more preferably 20 to 50mass %. When the content thereof is less than 5 mass %, the strength ofthe non-woven fabric base material sometimes becomes low. When it ismore than 60 mass %, melted components fill up the pores of thenon-woven fabric base material, and as a result, such cases sometimesoccur that the electrolyte retention of the non-woven fabric basematerial worsens or the resistance of the separator becomes high.

In view of achieving uniformity, the PET binder fiber (III) ispreferably a single-component type heat-fusible fiber.

The PET binder fiber (III) may contain an alkyl glycol and thederivative thereof as a copolymer component other than3,5-dicarbomethoxy benzene sulfonic acid. As the alkyl glycol and thederivative thereof, diethylene glycol is preferable.

It is preferred that the non-woven fabric base material (4) contains acrystallized PET fiber in addition to the PET binder fiber (III). Thecontent of the crystallized PET fiber is preferably 40 to 95 mass %,more preferably 45 to 90 mass %, and further more preferably 50 to 80mass %. When the content of the crystallized PET fiber is either lessthan 40 mass % or more than 95 mass %, the strength of the non-wovenfabric base material sometimes becomes low.

The average fiber diameter of the crystallized PET fiber is preferably0.5 to 10.0 μm, more preferably 0.7 to 8.0 μm, and further morepreferably 1.0 to 6.0 μm. When the average fiber diameter thereof isless than 0.5 μm, the fiber is so thin or fine that it sometimes fallsout of the non-woven fabric base material, and when it is more than 10.0μm, it becomes more difficult to make the separator thinner.

The fiber length of the crystallized PET fiber is preferably 1 to 10 mm,more preferably 2 to 7 mm, and further more preferably 3 to 5 mm. Whenthe fiber length thereof is less than 1 mm, the strength of thenon-woven fabric base material sometimes becomes low, and it is morethan 10 mm, the fibers tangle together and form lumps, and the thicknessof the base material sometimes becomes un-uniform.

The non-woven fabric base material (4) may contain other PET binderfiber than the PET binder fiber (III), and the content thereof ispreferably 20 mass % or less. When the content is more than 20 mass %,melted components of the PET binder fiber fill up the pores of thenon-woven fabric base material, and as a result, such cases sometimesoccur that the electrolyte retention of the non-woven fabric basematerial worsens or the resistance of the separator worsens.

The non-woven fabric base material (4) is composed mainly of a PETfiber. The term “composed mainly of a PET fiber” means that the contentof the PET fiber is 70 mass % or more. Further, other fibers than thePET fiber can be contained. For example, short fiber or fibrillatedmaterial of solvent-spun cellulose and regenerated cellulose; naturalcellulose fiber; pulped material or fibrillated material of naturalcellulose fiber; single fiber or composite fiber of polyolefin, acrylicresin, wholly aromatic polyester, wholly aromatic polyesteramide,polyamide, semi-aromatic polyamide, wholly aromatic polyamide, whollyaromatic polyether, wholly aromatic polycarbonate, wholly aromaticpolyazomethine, polyimides, polyamideimide (PAI), polyetheretherketone(PEEK), polyphenylene sulfide (PPS), poly-p-phenylenebenzo-bisoxazole(PBC)), polybenzimidazole (PBI), polytetrafluoroethylene (PTFE),ethylene-vinyl alcohol copolymer; and fibrillated material or dividedmaterial of single fiber or composite fiber consisting of these resins.One kind or two or more kinds of these fibers can be contained. The term“semi-aromatic” means that a chain such as fatty chain is partlycontained in a main chain. Wholly aromatic polyamide can be eitherpara-type or meta-type.

The basis weight of the non-woven fabric base material (4) is preferably6.0 to 20.0 g/m², more preferably 8.0 to 18.0 g/m², and further morepreferably 10.0 to 16.0 g/m². When it is more than 20.0 g/m², itsometimes becomes difficult to make the separator thinner, and when itis less than 6.0 g/m², it sometimes becomes difficult to obtainsufficient strength. The basis weight is measured based on the methoddefined in JIS P 8124 (Paper and board-Determination of basis weight).

The thickness of the non-woven fabric base material (4) is preferably 10to 30 μm, more preferably 13 to 27 μm, and further more preferably 15 to25 μm. When it is less than 10 μm, sufficiently high strength of thenon-woven fabric base material sometimes cannot be obtained. When it ismore than 30 μm, it sometimes becomes difficult to make the separatorthinner. The thickness of the non-woven fabric base material is measuredby an outside micrometer whose resolution is 0.001 mm, as defined in JISB7502-1994.

Non-Woven Woven Fabric Base Materials (1) to (4) for a Lithium IonSecondary Battery Separator, and a Lithium Ion Secondary BatterySeparator

The non-woven woven fabric base materials (1) to (4) of the presentinvention are preferably used for the production of a separator in whicha coating liquid containing an inorganic pigment has been coated ontothe non-woven fabric base material. In addition, the non-woven wovenfabric base materials (1) to (4) of the present invention may be usedfor the production of a separator in which a coating liquid containingan organic pigment has been coated onto the non-woven fabric basematerial; a separator in which a microporous resin film such aspolyethylene microporous film and polypropylene microporous film hasbeen laminated with the non-woven fabric base material; a separator inwhich a fine fiber layer has been formed on the non-woven fabric basematerial by an electrospinning method; and a separator in which a solidelectrolyte or a gel-like electrolyte has been coated on the non-wovenfabric base material. Like these, the non-woven fabric base materials(1) to (4) of the present invention are precursor sheets for a lithiumion secondary battery separator.

As the inorganic pigment, an inorganic oxide such as alumina, gibbsite,boehmite, magnesium oxide, magnesium hydroxide, silica, titanium oxide,barium titanate or zirconium oxide; an inorganic nitride such asaluminum nitride or silicon nitride; an aluminum compound; zeolite; andmica are exemplified.

As the organic particle, particles consisting of polyethylene,polypropylene, polyacrylonitrile, polymethyl methacrylate, polyethyleneoxide, polystyrene, polyvinylidene fluoride, an ethylene-vinyl monomercopolymer, polyolefin wax or the like are exemplified.

A medium for preparing a coating liquid containing an inorganic pigmentor an organic particle is not particularly limited as long as it canuniformly dissolve or disperse a binder, an inorganic pigment, anorganic particle and the like. For example, there can be used aromatichydrocarbons such as toluene; furans such as tetrahydrofuran; ketonessuch as methyl ethyl ketone; alcohols such as isopropyl alcohol;N-methyl-2-pyrrolidone (NMP); dimethylacetamide; dimethylformamide;dimethyl-sulfoxide; and water, as required. Further, these solvents maybe used in the form of a mixture thereof, as required. In addition, asthe medium to be used, the one does not swell or dissolve the non-wovenfabric base material is preferred.

As a method of coating a coating liquid containing an inorganic pigmentor an organic particle onto the non-woven fabric base material, forexample, various coating methods such as blade, rod, reverse-roll, lip,die, curtain and air-knife; various printing methods such asflexographic, screen, offset, gravure and inkjet; transfer methods suchas roll transfer and film transfer; and lifting methods such as dippingmay be selected and used accordingly.

A porous film is not particularly limited as long as it is afilm-formable resin. However, a polyolefin-based resin such aspolyethylene-based resin and polypropylene-based resin is preferable. Asthe polyethylene-based resin, a sole polyethylene-based resin such asultra-low density polyethylene, low density polyethylene, linear lowdensity polyethylene, medium density polyethylene, high densitypolyethylene and ultra-high density polyethylene is exemplified. Inaddition, an ethylene-propylene copolymer, a mixture of apolyethylene-based resin and another polyolefin resin or the like arealso exemplified. As the polypropylene-based resin, homo propylenepolymer (a propylene homopolymer), a random copolymer or a blockcopolymer of propylene and α-olefin such as ethylene, 1-butene,1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene and 1-decene areexemplified.

The lithium ion secondary battery in the present invention is a generalterm of a secondary battery in which a lithium ion in an electrolytesolution is responsible for electrical conduction. As a negativeelectrode active material of the battery, a carbon material such asnatural graphite, artificial graphite, hard carbon or coke; metalliclithium; an alloy of lithium with metal such as silicon, aluminum, tin,nickel or lead; a composite oxide of metal and lithium, such as lithiumtitanate, tin oxide or lithium silicate are exemplified. As a positiveelectrode active material, a composite oxide of transition metal andlithium, such as lithium cobalt oxide, lithium manganese oxide, lithiumnickel oxide, lithium titanate or lithium nickel manganese oxide;olivine-type lithium iron phosphate; a composite oxide of one or moretransition metals and lithium, or a composite oxide of one or moretransition metals, one or more typical metals and lithium, such as acomposite oxide of nickel-cobalt-manganese-lithium, a composite oxide ofnickel-cobalt-manganese-lithium, a composite oxide ofnickel-cobalt-aluminum-lithium, or a composite oxide ofiron-manganese-nickel-lithium are exemplified.

As an electrolyte solution of the battery, there can be used a solutionin which a lithium salt is dissolved in an organic solvent such aspropylene carbonate, ethylene carbonate, dimethyl carbonate, diethylcarbonate, dimethoxyethane, dimethoxymethane or a mixture thereof. Asthe lithium salt, lithium hexafluorophosphate (LiPF₆), lithiumtetrafluoroborate (LiBF₄) or the like are exemplified. Additives such asvinylene carbonate or boric acid esters may be added accordingly. Inaddition, there can be used a gelled electrolyte solution obtained bydissolving polymers such as polyethylene glycol or the derivativethereof, a polymethacrylate derivative, polysiloxane or the derivativethereof or polyvinylidene fluoride.

As a method of producing the non-woven fabric base materials (1) to (4),there can be used a method in which a non-woven fabric is produced byforming a fiber web, followed by adhering, fusing and entangling fibersinside the fiber web. The obtained non-woven fabric can be used eitheralone or in the form of a laminate consisting of plural sheets offabrics. As a method of producing a fiber web, a dry-laid method such asa carding method, an air-laid method, a spunbonding method or ameltblowing method; a wet-laid method such as a paper-making method; anelectrospinning method; or the like are exemplified. Of these, the fiberweb obtained by the wet-laid method is uniform and dense, and it can beused suitably as a non-woven fabric base material for a lithium ionsecondary battery separator. The wet-laid method is a method in whichfibers are dispersed into water to form a uniform paper-making slurry,then fiber web is obtained by using a paper-making machine having atleast one wire such as a cylinder wire, a Fourdrinier wire, aninclined-type wire and the like.

As a method of producing a non-woven fabric from a fiber web, there canbe used a water-jet entangling method, a needle-punching method, abinder-bonding method or the like. Especially, when the wet-laid methodmentioned above is used because of the uniformity, applying thebinder-bonding method to bond PET binder fiber is preferred. By thebinder-bonding method, a uniform non-woven fabric is formed from auniform fiber web.

The non-woven fabric produced like this is preferably pressed bycalendering or the like to adjust the thickness or to make the thicknessuniform. Especially, when a thin separator with a thickness of 25 μm orless is to be produced, it is preferred to press the non-woven fabric bycalendering or the like while heating it to make thin the thickness ofthe non-woven fabric base material. However, it is preferred that thenon-woven fabric base material is pressed under a temperature at whichthe PET binder fiber does not form film (a temperature lower by 20° C.or more than a melting point or a softening point of the PET binderfiber).

According to the present invention, it is possible to produce a lithiumion secondary battery separator with a thickness of 25 μm or less, whichin the prior art, has been difficult to produce with high productivitydue to wrinkles occurring during coating and the penetration-through ofa coating liquid. Especially, it is also possible to produce a lithiumion secondary battery separator with a thickness of 22 μm or less. Ofcourse, it is also possible to produce easily a lithium ion secondarybattery separator with a thickness of more than 25 μm. On the otherhand, an extremely thin separator with a thickness such as 10 μm or lessis difficult to produce even in the present invention. The thickness ofthe separator is measured by an outside micrometer whose resolution is0.001 mm, as defined in JIS B7502-1994.

EXAMPLES

The present invention will be explained more in detail by the followingexamples, but the present invention is not limited to these examples.

Examples 1-13, Comparative Examples 1-3 PET Binder Fiber A1

As a PET binder fiber, there was used a single-component type undrawnPET fiber (softening point of 120° C., melting point of 230° C.) with anaverage fiber diameter of 4.3 μm and a fiber length of 0.5 mm.

PET Binder Fiber A2

As a PET binder fiber A2, there was used a single-component type undrawnPET fiber (softening point of 120° C., melting point of 230° C.) with anaverage fiber diameter of 4.3 μm and a fiber length of 1.5 mm.

PET Binder Fiber A3

As a PET binder fiber A3, there was used a single-component type undrawnPET fiber (softening point of 120° C., melting point of 230° C.) with anaverage fiber diameter of 4.3 μm and a fiber length of 2.5 mm.

PET Binder Fiber A4

As a PET binder fiber A4, there was used a single-component type undrawnPET fiber (softening point of 120° C., melting point of 230° C.) with anaverage fiber diameter of 14.0 μm and a fiber length of 2.5 mm.

PET Binder Fiber A5

As a PET binder fiber A5, there was used a single-component type undrawnPET fiber (softening point of 120° C., melting point of 230° C.) with anaverage fiber diameter of 1.0 μm and a fiber length of 1.0 mm.

PET Binder Fiber A6

As a PET binder fiber A6, there was used a core-sheath type heat-fusiblePET fiber (melting point of sheath part: 110° C., core part: 250° C.)with an average fiber diameter of 7.2 μm and a fiber length of 2.0 mm.

PET Binder Fiber A7

As a PET binder fiber A7, there was used a single-component type undrawnPET fiber (softening point of 120° C., melting point of 230° C.) with anaverage fiber diameter of 4.3 μm and a fiber length of 0.3 mm.

PET Binder Fiber A8

As a PET binder fiber A8, there was used a single-component type undrawnPET fiber (softening point of 120° C., melting point of 230° C.) with anaverage fiber diameter of 4.3 μm and a fiber length of 3.0 mm.

PET Binder Fiber A9

As a PET binder fiber A9, there was used a single-component type undrawnPET fiber (softening point of 120° C., melting point of 230° C.) with anaverage fiber diameter of 15.0 μm and a fiber length of 2.5 mm.

In accordance with fiber materials and fiber blending ratios shown inTable 1, each paper-making slurry was prepared. In Table 1, “B1” means acrystallized PET fiber with an average fiber diameter of 2.5 μm and afiber length of 3 mm. “B2” means a crystallized PET fiber with anaverage fiber diameter of 3.2 μm and a fiber length of 3 mm. “B3” meansa crystallized PET fiber with an average fiber diameter of 5.5 μm and afiber length of 3 mm. “B4” means a crystallized PET fiber with anaverage fiber diameter of 7.8 μm and a fiber length of 5 mm. “C1” meansa wholly aromatic polyamide fiber(copoly(para-phenylene-3,4′-oxydiphenyleneterephthalic amide)) with afineness of 0.75 dtex and a fiber length of 3 mm. “C2” means an acrylicfiber (an acrylonitrile-based copolymer consisting of acrylonitrile,methyl acrylate, and a methacrylic acid derivative) with a fineness of0.10 dtex and a fiber length of 3 mm.

TABLE 1 Fiber blending ratio (mass %) PET binder fiber A1 A2 A3 A4 A5 A6A7 A8 A9 Average fiber diameter (μm) 4.3 4.3 4.3 14.0 1.0 7.2 4.3 4.315.0 Crystallized Fiber length (mm) PET fiber Others 0.5 1.5 2.5 2.5 1.02.0 0.3 3.0 2.5 B1 B2 B3 B4 C1 C2 Slurry 1 40 30 30 Slurry 2 40 30 30Slurry 3 40 30 30 Slurry 4 21 79 Slurry 5 25 75 Slurry 6 31 69 Slurry 745 55 Slurry 8 60 40 Slurry 9 50 40 10 Slurry 10 40 30 30 Slurry 11 3510 55 Slurry 12 40 30 30 Slurry 13 40 30 30 Slurry 14 40 60 Slurry 15 892 Slurry 16 61 39

Non-Woven Fabric Base Material Examples 1-3, 5-13

Each of slurries 1-3, 5-12 and 14 was paper-made by the wet-laid methodwith a cylinder wire and inclined wire combination paper making machineat a speed of 18 m/min to produce each of the non-woven fabric basematerials of Examples 1-3, 5-12 and 14 shown in Table 2. The thicknesswas adjusted by conducting a heat calendering treatment with a heatcalendering apparatus having metal roll-resin roll (Shore hardness D92)constitution under the conditions of a metal roll temperature of 195°C., a liner pressure of 200 kN/m, a processing speed of 10 m/min and 1nip.

Example 4

Slurry 4 was paper-made by the wet-laid method with a cylinder wire andinclined wire combination paper making machine at a speed of 18 m/min toproduce a separator of Example 4 shown in Table 2. The thickness wasadjusted by conducting a heat calendering treatment with a heatcalendering apparatus having metal roll-resin roll (Shore hardness D92)constitution under the conditions of a metal roll temperature of 195°C., a liner pressure of 100 kN/m, a processing speed of 10 m/min and 1nip.

Comparative Examples 1 and 3

Each of slurries 13 and 16 was paper-made by the wet-laid method with acylinder wire and inclined wire combination paper making machine at aspeed of 18 m/min to produce each of the separators of ComparativeExamples 1 and 2 shown in Table 2. The thickness was adjusted byconducting a heat calendering treatment with a heat calenderingapparatus having metal roll-resin roll (Shore hardness D92) constitutionunder the conditions of a metal roll temperature of 195° C., a linerpressure of 200 kN/m, a processing speed of 10 m/min and 1 nip.

Comparative Example 2

Slurry 15 was paper-made by the wet-laid method with a cylinder wire andinclined wire combination paper making machine at a speed of 18 m/min toproduce a separator of Comparative Example 2 shown in Table 2. Thethickness was adjusted by conducting a heat calendaring treatment with aheat calendering apparatus having metal roll-resin roll (Shore hardnessD92) constitution under the conditions of a metal roll temperature of195° C., a liner pressure of 100 kN/m, a processing speed of 10 m/minand 1 nip.

TABLE 2 Non-woven fabric base material Basis weight Thickness DensitySlurry (g/m²) (μm) (g/cm³) Exanple 1 1 10 15 0.67 Example 2 2 10 15 0.67Example 3 3 10 15 0.67 Example 4 4 12 25 0.48 Example 5 5 16 24 0.67Example 6 6 16 24 0.67 Example 7 7 16 24 0.67 Example 8 8 12 20 0.60Example 9 9 6 10 0.60 Example 10 10 20 30 0.67 Example 11 11 15 23 0.65Example 12 12 10 15 0.67 Comparative Example 1 13 10 15 0.67 Example 1314 10 15 0.67 Comparative Example 2 15 12 25 0.48 Comparative Example 316 12 20 0.60

[Tensile Strength]

Each of the non-woven fabric base materials of Examples and ComparativeExamples was cut into a rectangle with 50 mm width and 200 mm length sothat the long side thereof was along the flow direction, and each ofspecimens was elongated with a tabletop material testing instrument(trade name: STA-1150, supplied by Orientech Co., Ltd.) under theconditions of a grip distance of 100 mm and a tensile speed of 300ram/min. A load value at the break of the specimen was defined astensile strength. The tensile strength was measured at five or morepoints per specimen, and an average value of all values measured wasshown in Table 3.

[Electrolyte Retention]

Each of non-woven fabric base materials of Examples and ComparativeExamples was cut into a 100 mm×100 mm specimen, and the weight (W1)thereof was measured, then the specimen was immersed in propylenecarbonate for 1 minute, then hanged for 1 minute, and the weight (W2)thereof was measured, then electrolyte retention ratio was calculated bythe Formula 1 below.

Electrolyte retention ratio(%)=(W2−W1)/W1×100  (Formula 1)

The electrolyte retention ratio was measured 2 or more times perspecimen. When an average value of the measured values was 300% or more,the evaluation thereof was expressed as “A”. When said value was 270% ormore and less than 300%, it was expressed as “B”. When said value wasless than 270%, it was expressed as “C”.

[Wrinkle During Coating]

100 mass parts of boehmite with a volume average particle diameter of0.9 μm and a BET specific surface area of 5.5 m²/g was dispersed into150 mass parts of water, then 75 mass parts of an aqueous solutioncontaining 2 mass % of carboxymethyl cellulose sodium salt, in which theviscosity of an aqueous 1 mass % solution thereof at 25° C. was 200mPa·s, was added and stirred, then 10 mass parts of an emulsion (solidconcentration: 50 mass %) of carboxy-modified styrene-butadienecopolymer resin with a glass transition temperature of −18° C. and avolume average particle diameter of 0.2 μm was added and stirred, thenfinally adjusting water was added to adjust a solid concentration to 25mass % to produce a coating liquid A.

Onto the resin roll surface of each of the non-woven fabric basematerials of Examples and Comparative Examples, using a reverse gravurecoater as a coating machine, the coating liquid A was one side coated ata line speed of 30 m/min so that a coating amount of the liquid was 47g/m². The coated non-woven fabric base materials was dried by blowing90° C. hot air in a floating air dryer directly connected to the reversegravure coater to obtain a separator. An evaluation of “wrinkling duringcoating” was carried out by examining the occurrence of wrinkles in eachseparator observed when the separator was wound up to the 500 m lengthwith a reeler, and classifying it according to the following threecriteria.

∘: No wrinkles during coating were seen.

Δ: A few wrinkles during coating were seen.

×: Many wrinkles during coating were seen.

TABLE 3 Tensile Separator Wrinkling Strength Electrolyte Thicknessduring (N/m) retention (μm) coating Example 1 700 A 18 ◯ Example 2 764 A19 ◯ Example 3 730 A 19 Δ Example 4 695 A 26 Δ Example 5 983 A 26 ◯Example 6 1081 A 26 ◯ Example 7 1222 A 27 ◯ Example 8 831 B 25 ◯ Example9 682 A 11 ◯ Example 10 1460 A 30 ◯ Example 11 1120 A 25 ◯ Example 12590 A 20 ◯ Comparative Example 1 719 A 19 X Example 13 585 A 17 ◯Comparative Example 2 380 A 28 X Comparative Example 3 836 C 27 ◯

Each of the non-woven fabric base materials produced in Examples 1-13shown in Table 3 is corresponding to the non-woven fabric base material(1), since it is composed mainly of a PET fiber and comprises a PETbinder fiber and a crystallized PET fiber, and the content of a PETbinder fiber with a fiber length of 2.5 mm or less is 10 to 60 massa.Thus, since each of the non-woven fabric base materials of Examples 1-13has a small elongation and has the uniform distribution of the PETbinder fiber therein, the wrinkling during coating could be prevented,and a lithium ion secondary battery separator in which an inorganicpigment was coated onto the non-woven fabric base material could beproduced with high productivity.

Each of the non-woven fabric base materials produced in Examples 1-11 isalso corresponding to the non-woven fabric base material (2), which iscomposed mainly of a PET fiber and contains 21 to 60 mass % of the PETbinder fiber (I) with an average fiber diameter of 14.0 μm or less and afiber length of 0.5 to 2.5 mm. Thus, since each of the non-woven fabricbase materials of Examples 1-11 has a small elongation and has theuniform distribution of the PET binder fiber therein, the wrinklingduring coating can be prevented more effectively compared with thenon-woven fabric base materials produced in Examples 12 and 13, and alithium ion secondary battery separator in which an inorganic pigmentwas coated onto the non-woven fabric base material could be producedwith higher productivity. In the non-woven fabric base material producedin Example 12, the fiber length of the PET binder fiber was less than0.5 mm, and as a result, the fiber slightly fell out of the non-wovenfabric base material and the tensile strength of the non-woven fabricbase material became slightly low compared with those of the non-wovenfabric base materials produced in Examples 1-11. In the non-woven fabricbase material produced in Example 13, the average fiber diameter of thePET binder fiber was more than 14.0 μm, and as a result, the number ofthe fiber in the thickness direction decreased and the tensile strengthof the non-woven fabric base material became low compared with those ofthe non-woven fabric base materials produced in Examples 1-11.

In the non-woven fabric base material produced in Comparative Example 1,the fiber length of the PET binder fiber was more than 2.5 mm, and as aresult, not only the non-woven fabric base material was liable toelongate, but also the PET binder fibers tangled together. Thus,wrinkling likely occurred and the productivity thereof was low.

With the non-woven fabric base material produced in Comparative Example2 having the PET binder fiber content of less than 10 mass %, wrinklingduring coating likely occurred and the productivity of the separator waslow. The non-woven fabric base material produced in Comparative Example3 had the PET binder fiber content of more than 60 mass %, and as aresult, melted components fill up fine pores and the electrolyteretention worsened.

Hereafter, the non-woven fabric base materials produced in Examples 1-11are compared. The non-woven fabric base material produced in Example 1had a slightly small fiber length of the PET binder fiber, the non-wovenfabric base material produced in Example 4 had a slightly large averagefiber diameter of the PET binder fiber, and the non-woven fabric basematerial produced in Example 9 had a slightly small basis weight. As aresult, the non-woven fabric base materials produced in Examples 1, 4and 9 had slightly low tensile strengths compared with the non-wovenfabric base materials produced in Examples 2, 3, 5-8, 10 and 11.

The non-woven fabric base material produced in Example 8 has a slightlylarge content of the PET binder fiber (I) with an average fiber diameterof 14.0 μm or less and a fiber length of 0.5 to 2.5 mm, the electrolyteretention thereof is slightly inferior to those of the non-woven fabricbase materials produced in Examples 1-7 and 9-11.

The non-woven fabric base material produced in Example 3 had a slightlylarge fiber length of the PET binder fiber, and the non-woven fabricbase material produced in Example 4 had a slightly smaller content ofthe PET binder fiber with an average fiber diameter of 14.0 μm or lessand a fiber length of 0.5 to 2.5 mm, and as a result, the wrinkleformation during coating was relatively easy to occur compared with thenon-woven fabric base materials produced in Examples 1, 2 and 5-11.

Examples 14-29, Comparative Examples 4 and 5 PET Binder Fiber (AverageFiber Diameter of 1.3 μm)

A binder fiber with an average fiber diameter of 1.3 μm was produced bycutting filaments produced by eluting a sea component from a sea-islandtype fiber, to a predetermined length.

PET Binder Fiber (Average Fiber Diameter of 1.6 μm)

A binder fiber with an average fiber diameter of 1.6 μm was produced bycutting filaments produced by eluting a sea component from a sea-islandtype fiber, to a predetermined length.

PET Binder Fiber (Average Fiber Diameter of 2.8 μm)

A binder fiber with an average fiber diameter of 2.8 μm was produced bycutting filaments produced by eluting a sea component from a sea-islandtype fiber, to a predetermined length.

PET Binder Fiber (Average Fiber Diameter of 4.3 μm)

A binder fiber with an average fiber diameter of 4.3 μm was produced bycutting filaments produced by a melt-spinning method, to a predeterminedlength.

[Crystallized PET Fiber 24]

As a drawn crystallized PET fiber 24, there was used a crystallized PETstaple (softening point of 250° C.) which was produced by cuttingfilaments with an average fiber diameter of 2.4 μm produced by amelt-spinning method, to a length of 3.0 mm.

[Crystallized PET Fiber 16]

As a drawn crystallized PET fiber 16, there was used a crystallized PETstaple (softening point of 250° C.) which was produced by cuttingfilaments with an average fiber diameter of 1.6 μm produced by eluting asea component from a sea-island type fiber, to a length of 3.0 mm.

[Crystallized PET Fiber 43]

As a drawn crystallized PET fiber 43, there was used a crystallized PETstaple (softening point of 250° C.) which was produced by cuttingfilaments with an average fiber diameter of 4.3 μm produced by a meltspinning method, to a length of 3.0 mm.

[Cellulose Fiber]

A lyocell (solvent-spun cellulose) fiber which was beaten to a Canadianstandard freeness of 50 ml with a double disk refiner was used.

[Aramid Fiber]

A para-aramid fiber which was beaten to a Canadian standard freeness of250 ml with a double disk refiner was used.

[Paper-Making]

In accordance with fiber materials and fiber blending ratios as shown inTables 4 and 5, paper-making slurries were prepared. Each slurry wasmade into paper at a speed of 8 m/min with an inclined-type wire papermaking machine so that the basis weight after drying was 9.0 g/m²,followed by drying with a cylinder dryer, and then heat calendering witha heat calendering apparatus having metal roll-resin roll (Shorehardness 992) constitution under the conditions of a metal rolltemperature of 195° C., a liner pressure of 100 kN/m, a processing speedof 5 m/min and 1 nip, to obtain a non-woven fabric base material with athickness of 13 μm.

TABLE 4 Example Example Example 14 Example 15 Example 16 Example 17Example 18 Example 19 20 21 Binder fiber Fiber 1.6 2.8 2.8 2.8 2.8 2.82.8 2.8 diameter (μm) Fiber 1.5 1.5 1.0 2.5 1.5 1.5 1.5 1.5 length (mm)Content 20 20 20 20 10 30 20 20 (mass %) Crystallized Content 80 80 8080 90 70 60 60 PET fiber24 (mass %) Crystallized PET fiber16Crystallized PET fiber43 Cellulose fiber 20 Aramid fiber 20 Tensile(N/m) 570 560 520 510 480 600 470 450 strength Separator (μm) 19 18 1818 17 20 22 21 thickness Penetration-through of B B B B B A A A coatingliquid Internal (Ω) 4.2 4.1 4.1 4.2 4.0 4.4 4.3 4.3 resistance Wrinkle ◯◯ Δ ◯ ◯ ◯ ◯ ◯

TABLE 5 Example Example Comparative Example Example Comparative ExampleExample Example Example 22 23 Example 4 24 25 Example 5 26 27 28 29Binder fiber Fiber 1.3 4.3 2.8 2.8 2.8 2.8 2.8 2.8 2.8 2.8 diameter (μm)Fiber 1.5 1.5 1.5 1.5 0.5 3.0 2.8 1.5 1.5 1.5 length (mm) Content 30 305 40 30 30 30 30 20 20 (mass %) Crystallized Content 70 70 95 60 70 7040 40 PET fiber24 (mass %) Crystallized 80 PET fiber16 Crystallized 80PET fiber43 Cellulose 30 fiber Aramid fiber 30 Tensile (N/m) 400 420 330620 330 350 300 280 280 280 strength Separator (μm) 19 17 15 17 18 18 2220 20 16 thickness Penetration-through of B C C B C C B B A D coatingliquid Internal (Ω) 4.3 4.3 4.1 4.8 4.1 4.3 4.4 4.6 4.8 4.5 resistanceWrinkling ◯ ◯ X ◯ Δ X ◯ ◯ ◯ ◯

[Tensile Strength]

Each of the non-woven fabric base materials of Examples and ComparativeExamples was cut into a rectangle with 50 mm width and 200 mm length sothat the long side thereof was along the flow direction, and each ofspecimens was elongated with a tabletop material testing instrument(trade name: STA-1150, supplied by Orientech Co., Ltd.) under theconditions of a grip distance of 100 mm and a tensile speed of 300mm/min. A load value at the break of the specimen was defined as tensilestrength. The result was shown in Tables 4 and 5.

[Wrinkling]

To simulate wrinkling during coating, the following experiment wascarried out. That is, each of the non-woven fabric base materials inExamples and Comparative Examples was slit to a width of 250 mm and sentout at a unwinding speed of 3 m/min and a unwinding tension of 5 N, andthereafter it was changed its direction into a horizontal direction witha roll provided vertically above 2 m, and was finally wound up. Theoccurrence of wrinkles when the separator was wound up to 20 m lengthwas examined and classified according to the following criteria.

∘: No wrinkles occurred.

Δ: Wrinkles sometimes occurred, but disappeared when horizontal tensionwas applied.

×: Wrinkles sometimes occurred, and immediately re-ocurred even whenhorizontal tension was applied.

[Separator]

Onto each of the non-woven fabric base materials in Examples andComparative Examples, a coating liquid with a non-volatile content of 40mass % was coated with a rod coater so that the coating amount afterdrying was 10 g/m². The coating liquid contained 100 mass parts ofmagnesium hydroxide with an average particle diameter of 1.0 μm, 1.5mass parts of styrene-butadiene latex and 1.0 mass part of carboxymethylcellulose sodium. Here, a black-colored drawing sheet was used as anunderlay.

[Penetration-Through of a Coating Liquid]

In the above-mentioned separator production, the amount of the coatingliquid which penetrated through the non-woven fabric base material andthereafter adhered to the black-colored drawing sheet used as anunderlay was examined and classified according to the following fourcriteria. The results are shown in Tables 4 and 5.

A No coating liquid adhered to the mount sheet.

B The coating liquid adhered in dots to the mount sheet (adhesion areaof less than 10%).

C The penetrated-through coating liquid adhered to the mount sheet(adhesion area of 10% to 30%).

D A large amount of the penetrated-through coating liquid adhered to themount sheet (adhesion area of more than 30%).

[Internal Resistance]

Using each separator produced, there was produced a lithium ionsecondary battery for evaluation with a capacity of 30 mAh (electrodearea: 15 cm², positive electrode: lithium manganese oxide, negativeelectrode: hard carbon, electrolyte solution: mixed solvent solution of1M lithium hexafluorophosphate (LiPF₆) in ethylene carbonate(EC)/diethyl carbonate (DEC) in a volume ratio of 3/7, pouch-typebattery). Based on a terminal voltage E₀ after complete charging and aterminal voltage E₁ immediately after discharging for 10 seconds at 150mA, the internal resistance was calculated by the following Formula 2.The results were shown in Tables 4 and 5.

R ₁=(E ₀ −E ₁)/0.15  (Formula 2)

Each of the non-woven fabric base materials produced in Examples 14-30is corresponding to the non-woven fabric base material (1) which iscomposed mainly of a PET fiber, contains a PET binder fiber and acrystallized PET fiber, and has a content of 10 to 60 mass % of a PETbinder fiber with a fiber length of 2.5 mm or less. Thus, wrinkles werenot formed during coating or, even if formed, they could be corrected byadjusting horizontal tension. On the contrary, the non-woven fabric basematerial of Comparative Example 4 contained only 5 mass % of the PETbinder fiber and the non-woven fabric base material of ComparativeExample 5 contained the PET binder fiber with a long fiber length of 3.0mm, and as a result, wrinkles were formed and were hard to be corrected.

Each of the non-woven fabric base materials produced in Examples 14-21is also corresponding to the non-woven fabric base material (3) in whichthe content of the PET binder fiber (II) with an average fiber diameterof 1.5 to 2.8 μm and a fiber length of 1.0 to 2.5 mm was 10 to 30 massa,the total content of the PET binder fiber (II) and the crystallized PETfiber was 80 to 100 mass %, and the average fiber diameter of thecrystallized PET fiber was 2.0 to 4.0 μm. Thus, the tensile strength washigh, the penetration-through of a coating liquid was little duringcoating an inorganic pigment on these non-woven fabric base materials,and the internal resistance of the lithium ion secondary batteryseparator was as low as 4.4 Ω.

Since the non-woven fabric base material of Example 22 had a small fiberdiameter of 1.3 μm of the PET binder fiber, the tensile strength was 400N/m, which was low compared with those of the non-woven fabric basematerials produced in Examples 14-21. Since the non-woven fabric basematerial of Example 23 had a large fiber diameter of 4.3 μm of the PETbinder fiber, the tensile strength was 420 N/m, which was low comparedwith those of the non-woven fabric base materials produced in Examples14-21. Since the non-woven fabric base material of Example 24 had alarge content of 40 mass % of the PET binder fiber, the internalresistance of the lithium ion secondary battery separator produced byusing it was 4.8Ω, which was high compared with those of the lithium ionsecondary battery separators produced from the non-woven fabric basematerials of Examples 14-21. Since the non-woven fabric base material ofExample 25 had a small fiber length of 0.5 mm of the PET binder fiber,the tensile strength was 300 N/m, which was low compared with those ofthe non-woven fabric base materials of Examples 14-21. Since thenon-woven fabric base materials of Examples 26 and 27 contained morethan 20 mass % of the fiber other than the PET fiber, their tensilestrengths were 300 N/m and 280 N/m, respectively, which were lowcompared with those of the non-woven fabric base materials of Examples14-21. Since the non-woven fabric base material of Example 28 had asmall fiber diameter of 1.6 μm of the crystallized PET fiber, theinternal resistance of the lithium ion secondary battery separatorproduced by using it was 4.8 0, which was high compared with those ofthe lithium ion secondary battery separators produced by using thenon-woven fabric base materials of Examples 14-21. Since the non-wovenfabric base material of Example 29 had a large fiber diameter of 4.3 μmof the crystallized PET fiber, the penetration-through of a coatingliquid was more noticeable compared with the non-woven fabric basematerials of Examples 14-21.

Examples 30-44, Comparative Examples 6-8 PET Binder Fiber A11

As a PET binder fiber A11, there was used a single-component typeundrawn PET fiber (softening point of 120° C., melting point of 230° C.)with an average fiber diameter of 0.5 μm and a fiber length of 0.5 mm,which contained 3,5-dicarbomethoxy benzene sulfonic acid and diethyleneglycol as copolymer components.

PET Binder Fiber A12

As a PET binder fiber A12, there was used a single-component typeundrawn PET fiber (softening point of 120° C., melting point of 230° C.)with an average fiber diameter of 1.0 μm and a fiber length of 1.0 mm,which contained 3,5-dicarbomethoxy benzene sulfonic acid and diethyleneglycol as copolymer components.

PET Binder Fiber A13

As a PET binder fiber A13, there was used a single-component typeundrawn PET fiber (softening point of 120° C., melting point of 230° C.)with an average fiber diameter of 2.0 μm and a fiber length of 2.0 mm,which contained 3,5-dicarbomethoxy benzene sulfonic acid and diethyleneglycol as copolymer components.

PET Binder Fiber A14

As a PET binder fiber A14, there was used a single-component typeundrawn PET fiber (softening point of 120° C., melting point of 230° C.)with an average fiber diameter of 4.3 μm and a fiber length of 3.0 mm,which contained 3,5-dicarbomethoxy benzene sulfonic acid and diethyleneglycol as copolymer components.

PET Binder Fiber A15

As a PET binder fiber A15, there was used a single-component typeundrawn PET fiber (softening point of 120° C., melting point of 230° C.)with an average fiber diameter of 10.0 μm and a fiber length of 4.0 mm,which contained 3,5-dicarbomethoxy benzene sulfonic acid and diethyleneglycol as copolymer components.

PET Binder Fiber A16

As a PET binder fiber A16, there was used a single-component typeundrawn PET fiber (softening point of 120° C., melting point of 230° C.)with an average fiber diameter of 14.0 μm and a fiber length of 5.0 mm,which contained 3,5-dicarbomethoxy benzene sulfonic acid and diethyleneglycol as copolymer components.

PET Binder Fiber a17

As a PET binder fiber a17, there was used a single-component typeundrawn PET fiber (softening point of 120° C., melting point of 230° C.)with an average fiber diameter of 10.5 μm and a fiber length of 5.0 mm,which contained diethylene glycol as copolymer components.

PET Binder Fiber a18

As a PET binder fiber a18, there was used a core-sheath typeheat-fusible PET fiber (melting point of sheath part: 110° C., corepart: 250° C.) with an average fiber diameter of 10.1 μm and a fiberlength of 5.0 mm, which contained diethylene glycol as a copolymercomponent.

In accordance with fiber materials and fiber blending ratios as shown inTable 6, each paper-making slurry was prepared. In Table 6, “B11” meansa crystallized PET fiber with an average fiber diameter of 0.7 μm and afiber length of 1.7 mm, which contained diethylene glycol as a copolymercomponent. “B12” means a crystallized PET fiber with an average fiberdiameter of 2.5 μm and a fiber length of 3.0 mm, which containeddiethylene glycol as a copolymer component. “B13” means a crystallizedPET fiber with an average fiber diameter of 3.2 μm and a fiber length of3.0 mm, which contained diethylene glycol as a copolymer component.“B14” means a crystallized PET fiber with an average fiber diameter of5.5 μm and a fiber length of 3.0 mm, which contained diethylene glycolas a copolymer component. “B15” means a crystallized PET fiber with anaverage fiber diameter of 7.8 μm and a fiber length of 5 mm, whichcontained diethylene glycol as a copolymer component. “C11” means awholly aromatic polyamide fiber(copoly(para-phenylene-3,4′-oxydiphenyleneterephthalic amide)) with afineness of 0.75 dtex and a fiber length of 3 mm.

TABLE 6 Fiber blending ratio Slurry (mass %) 17 A11/B13 = 40/60 18A12/B13 = 40/60 19 A13/B13 = 40/60 20 A14/B13 = 40/60 21 A15/B13 = 40/6022 A16/B13 = 40/60 23 A14/B11/B12 = 5/10/85 24 A14/B13/B14 = 10/45/45 25A14/B13/B15 = 20/40/40 26 A14/B13 = 50/50 27 A14/B13 = 55/45 28 A14/B13= 60/40 29 A14/B13/C11 = 40/30/30 30 A14/a17/B13 = 10/20/70 31A14/a18/B13 = 20/10/70 32 a17/B13 = 40/60 33 a18/B13 = 40/60

Non-Woven Fabric Base Material Examples 30-44

Slurries 17-31 were paper-made by the wet-laid method with a cylinderwire and inclined wire combination paper making machine at a speed of 18m/min to produce the non-woven fabric base materials of Examples 30-44shown in Table 7. The thickness thereof was adjusted by conducting aheat calendering treatment with a heat calendering apparatus havingmetal roll-resin roll (Shore hardness D92) constitution under theconditions of a metal roll temperature of 195° C., a liner pressure of200 kN/m, a processing speed of 10 m/min and 1 nip.

Comparative Example 6

Slurry 32 was paper-made by the wet-laid method with a cylinder wire andinclined wire combination paper making machine at a speed of 18 m/min toproduce the non-woven fabric base material of Comparative Example 6shown in Table 7. The thickness thereof was adjusted by conducting aheat calendering treatment with a heat calendering apparatus havingmetal roll-resin roll (Shore hardness D92) constitution under theconditions of a metal roll temperature of 195° C., a liner pressure of200 kN/m, a processing speed of 10 m/min and 1 nip.

Comparative Example 7

Slurry 33 was paper-made by the wet-laid method with a cylinder wire andinclined wire combination paper making machine at a speed of 18 m/min toproduce the non-woven fabric base material of Comparative Example 7shown in Table 7.

Comparative Example 8

The non-woven fabric base material of Comparative Example 7 wassubjected to a heat calendering treatment with a heat calenderingapparatus having metal roll-resin roll (Shore hardness D92) constitutionunder the conditions of a metal roll temperature of 195° C., a linerpressure of 200 kN/m, a processing speed of 10 m/min and 1 nip toproduce the non-woven fabric base material of Comparative Example 8shown in Table 7.

TABLE 7 Basis weight Thickness Density Slurry (g/m²) (μm) (g/cm³)Example 30 17 10 15 0.67 Example 31 18 10 15 0.67 Example 32 19 10 150.67 Example 33 20 10 15 0.67 Example 34 21 10 16 0.63 Example 35 22 1016 0.63 Example 36 23 6 10 0.60 Example 37 24 8 13 0.62 Example 38 25 1016 0.63 Example 39 26 16 25 0.64 Example 40 27 18 27 0.67 Example 41 2820 30 0.67 Example 42 29 12 18 0.67 Example 43 30 12 18 0.67 Example 4431 12 18 0.67 Comparative Example 6 32 10 16 0.63 Comparative Example 733 10 30 0.33 Comparative Example 8 33 10 16 0.63

[Tensile Strength]

Each of the non-woven fabric base materials of Examples and ComparativeExamples was cut into a rectangle with 50 mm width and 200 mm length sothat the long side thereof was along the flow direction, and each ofspecimens was elongated with a tabletop material testing instrument(trade name: STA-1150, supplied by Orientech Co., Ltd.) under theconditions of a grip distance of 100 mm and a tensile speed of 300mm/min. A load value at the break of the specimen was defined as tensilestrength. The tensile strength was measured at five or more points perspecimen, and an average value of all values measured was calculated.When the tensile strength was 700 N/m or more, the evaluation thereofwas expressed as “A”. When the tensile strength was 600 N/m or more andless than 700 N/m, it was expressed as “B”. When tensile strength wasless than 600 N/m, it was expressed as “C”. The results were shown inTable 8.

[Electrolyte Retention]

Each of the non-woven fabric base materials of Examples and ComparativeExamples was cut into a 100 mm×100 mm specimen, and the weight (W1)thereof was measured, then the specimen was immersed in propylenecarbonate for 1 minute, then hanged for 1 minute, and the weight (W2)thereof was measured, then electrolyte retention ratio was 1.5calculated by the Formula 1 below.

Electrolyte retention ratio(%)=(W2−W1)/W1×100  (Formula 1)

The electrolyte retention ratio was measured 2 or more times perspecimen. When an average value of the measured values was 300% or more,the evaluation thereof was expressed as “A”. When said value was 270% ormore and less than 300%, it was expressed as “B”. When said value wasless than 270%, it was expressed as “C”.

[Production of a Separator]

100 mass parts of boehmite with a volume average particle diameter of0.9 μm and a BET specific surface area of 5.5 m²/g was dispersed into150 mass parts of water to prepare a dispersion, to which was added 75mass parts of an aqueous solution containing 2 massa of carboxymethylcellulose sodium salt, in which the viscosity of an aqueous 1 mass %solution thereof at 25° C. was 200 mPa·s, followed by stirring. To theresulting mixture was added 10 mass parts of an emulsion (solidconcentration: 50 mass %) of carboxy-modified styrene-butadienecopolymer resin with a glass transition temperature of −18° C. and avolume average particle diameter of 0.2 μm, followed by stirring.Finally water was added to adjust a solid concentration to 25 mass % toproduce a coating liquid A.

Using a reverse-type gravure coater as a coating machine, the coatingliquid A was one side coated onto the resin roll surface of each of thenon-woven fabric base materials of Examples and Comparative Examples ata line speed of 30 m/min so that a coating amount of the liquid was 47g/m². Each of coated non-woven fabric base materials was dried byblowing hot air of 90° C. thereto with a floating air dryer directlyconnected to the reverse-type gravure coater, to produce a separator.

[Production of a Battery for Evaluation]

Using each separator produced, there was produced a battery forevaluation with a capacity of 30 mAh, in which a positive electrode waslithium manganese oxide, a negative electrode was mesocarbon microbeadsand an electrolyte solution was a mixed solvent solution of 1 mol/L oflithium hexafluorophosphate (LiPF₆) in diethylene carbonate(DEC)/ethylene carbonate (EC) in a volume ratio of 3/7.

[Evaluation of Internal Resistance]

Each battery produced was firstly subjected to the running-in chargingand discharging of 5 cycles under the sequence of “constant electriccurrent charging of 60 mA→constant electric voltage charging of 4.2 v (1hour)→constant electric current discharging of 60 mA→toward next cycleafter reaching 2.8 v”. Next, it was subjected to the charging anddischarging under the sequence of “constant electric current charging of60 mA→constant electric voltage charging of 4.2 v (1 hour)→constantelectric current discharging of 6 mA for 30 minutes (discharging amountof 3 mAh)”, followed by measuring a voltage (voltage a) just before theend of the discharging. Then, it was subjected to the charging anddischarging under the sequence of “constant electric current charging of60 mA→constant electric voltage charging of 4.2 v (1 hour)→constantelectric current discharging of 90 mA for 2 minutes (The dischargingamount of 3 mAh)” followed by measuring a voltage (voltage b) justbefore the end of the discharging. Based on the measured voltages a andb, each internal resistance was calculated by the following Formula 3.

Internal resistance Ω=(Voltage a−Voltage b)/(90 mA−6 mA)  (Formula 3)

A: Internal resistance was less than 4ΩB: Internal resistance was 4Ω or more to less than 5ΩC: Internal resistance was 5Ω or more

TABLE 8 Tensile Separator Internal strength Electrolyte Thicknessresistance (N/m) retention (μm) (Ω) Example 30 B A 22 A Example 31 A A21 A Example 32 A A 20 A Example 33 A A 18 A Example 34 A A 18 A Example35 B A 17 A Example 36 B A 15 A Example 37 A A 15 A Example 38 A A 18 AExample 39 A A 31 A Example 40 A A 34 A Example 41 A B 38 B Example 42 AA 26 A Example 43 A B 23 B Example 44 A A 22 A Comparative Example 6 A C18 C Comparative Example 7 C A 30 A Comparative Example 8 A C 17 C

Each of the non-woven fabric base materials of Examples 30-44 shown inTable 8 is corresponding to the non-woven fabric base material (4) whichis composed mainly of a PET fiber and contains 3,5-dicarbomethoxybenzene sulfonic acid as a copolymer component. Thus, the strength ofthe non-woven fabric base material was high, the electrolyte retentionwas excellent and the resistance of the separator was extremely low.

On the other hand, the non-woven fabric base material produced inComparative Example 6 did not contain the PET binder fiber containing3,5-dicarbomethoxy benzene sulfonic acid as a copolymer component, andas a result, the binder fibers fill up the pores inside the non-wovenfabric base material and the electrolyte retention worsened. Further,the resistance of the separator became high.

Each of the non-woven fabric base materials produced in ComparativeExamples 7 and 8 did not contain the PET binder fiber containing3,5-dicarbomethoxy benzene sulfonic acid as a copolymer component, andas a result, the non-woven fabric base material of Comparative Example 7had lower strength before the heat calendering treatment, and thenon-woven fabric base material of Comparative Example 8 had worseelectrolyte retention and higher resistance of the separator after theheat calendering treatment.

The non-woven fabric base material produced in Example 30 had a slightlysmall fiber diameter and a slightly small fiber length of the PET binderfiber containing 3,5-dicarbomethoxy benzene sulfonic acid as a copolymercomponent, and as a result, it was observed that the fibers slightlyfell out of the non-woven fabric base material, and the tensile strengthbecame slightly low compared with those of the non-woven fabric basematerials of Example 31-34 and 37-44.

The non-woven fabric base material produced in Example 35 had a slightlylarge fiber diameter of the PET binder fiber containing3,5-dicarbomethoxy benzene sulfonic acid as a copolymer component, andas a result, the number of the fiber in the thickness direction slightlydecreased and the tensile strength became slightly low compared withthose of the non-woven fabric base materials produced in Examples 31-34and 37-44.

The non-woven fabric base material produced in Example 36 had a slightlysmall content of PET binder fiber containing 3,5-dicarbomethoxy benzenesulfonic acid as a copolymer component and a slightly high basis weightthereof, and as a result, the tensile strength became low compared withthose of the non-woven fabric base materials of Examples 31-34 and37-44.

The non-woven fabric base material produced in Example 41 had a slightlylarge content of PET binder fiber containing 3,5-dicarbomethoxy benzenesulfonic acid as a copolymer component, and as a result, filled parts ofpores were observed inside the non-woven fabric base material and theelectrolyte retention became slightly low and the resistance of theseparator became slightly high compared with those of the non-wovenfabric base materials of Examples 30-40, 42 and 44.

In the non-woven fabric base material produced in Example 43, both ofthe PET binder fiber containing 3,5-dicarbomethoxy benzene sulfonic acidas a copolymer component and the PET binder fiber not containing3,5-dicarbomethoxy benzene sulfonic acid as a copolymer component wereused. However, since the content of the PET binder fiber not containing3,5-dicarbomethoxy benzene sulfonic acid as a copolymer component wasslightly larger, filled parts of pores were observed inside thenon-woven fabric base material, and as a result, the electrolyteretention became slightly low and the resistance of the separator becameslightly high compared with those of the non-woven fabric base materialsin Examples 30-40, 42 and 44.

INDUSTRIAL UTILITY

As an example in which the non-woven fabric base material of the presentinvention is utilized, a lithium ion secondary battery separator issuitable.

1. A non-woven fabric base material for a lithium ion secondary batteryseparator composed mainly of a polyethylene terephthalate fiber,characterized in that: the non-woven fabric base material comprises apolyethylene terephthalate binder fiber and a crystallized polyethyleneterephthalate fiber, and the content of a polyethylene terephthalatebinder fiber having a fiber length of 2.5 mm or less is 10 to 60 mass %.2. The non-woven fabric base material for a lithium ion secondarybattery separator recited in claim 1, wherein the content of apolyethylene terephthalate binder fiber having an average fiber diameterof 14.0 μm or less and a fiber length of 0.5 to 2.5 mm is 21 to 60 mass%.
 3. The non-woven fabric base material for a lithium ion secondarybattery separator recited in claim 1, wherein the content of apolyethylene terephthalate binder fiber having an average fiber diameterof 1.5 to 2.8 μm and a fiber length of 1.0 to 2.5 mm is 10 to 30 mass %;the total content of the polyethylene terephthalate binder fiber and thecrystallized polyethylene terephthalate fiber is 80 to 100 mass %; andthe average fiber diameter of crystallized polyethylene terephthalatefiber is 2.0 to 4.0 μm.
 4. A non-woven fabric base material for alithium ion secondary battery separator composed mainly of apolyethylene terephthalate fiber, characterized in that: the non-wovenfabric base material comprises a polyethylene terephthalate binder fibercontaining 3,5-dicarbomethoxy benzene sulfonic acid as a copolymercomponent.
 5. A lithium ion secondary battery separator, produced bysubjecting the non-woven fabric base material for a lithium ionsecondary battery separator recited in claim 1 to at least one treatmentselected from: a treatment in which a coating liquid containing aninorganic pigment is coated; a treatment in which a coating liquidcontaining an organic particle is coated; a treatment in which amicroporous resin film is laminated; a treatment in which a fine fiberlayer is formed by an electrospinning method; and a treatment in which asolid electrolyte or a gel-like electrolyte is coated.
 6. A lithium ionsecondary battery separator, produced by subjecting the non-woven fabricbase material for a lithium ion secondary battery separator recited inclaim 2 to at least one treatment selected from: a treatment in which acoating liquid containing an inorganic pigment is coated; a treatment inwhich a coating liquid containing an organic particle is coated; atreatment in which a microporous resin film is laminated; a treatment inwhich a fine fiber layer is formed by an electrospinning method; and atreatment in which a solid electrolyte or a gel-like electrolyte iscoated.
 7. A lithium ion secondary battery separator, produced bysubjecting the non-woven fabric base material for a lithium ionsecondary battery separator recited in claim 3 to at least one treatmentselected from: a treatment in which a coating liquid containing aninorganic pigment is coated; a treatment in which a coating liquidcontaining an organic particle is coated; a treatment in which amicroporous resin film is laminated; a treatment in which a fine fiberlayer is formed by an electrospinning method; and a treatment in which asolid electrolyte or a gel-like electrolyte is coated.
 8. A lithium ionsecondary battery separator, produced by subjecting the non-woven fabricbase material for a lithium ion secondary battery separator recited inclaim 4 to at least one treatment selected from: a treatment in which acoating liquid containing an inorganic pigment is coated; a treatment inwhich a coating liquid containing an organic particle is coated; atreatment in which a microporous resin film is laminated; a treatment inwhich a fine fiber layer is formed by an electrospinning method; and atreatment in which a solid electrolyte or a gel-like electrolyte iscoated.